Signals may be represented mathematically as functions of time and, more particularly as either analog (continuous-time) signals or digital (discrete-time) signals. As used herein, analog signals are defined for a continuum of time values and may be represented, for example, by a continuously time-varying electrical voltage. Digital signals, in contrast, are defined only at discrete points in time and may, for example, be represented by a series of electrical voltage pulses. Each represents a bit (i.e., a 0 or a 1), and accordingly, the series of pulses or bits is described as a bitstream, or a series of 0's and 1's. The bitstream may represent data or it may represent, numerically, the strength or amplitude, of a continuous signal at discrete points in time.
It is useful in many applications to combine analog and digital signals into a single composite signal. Furthermore, there is a particular need in numerous applications to combine an analog signal representing audio, with a digital signal representing data. For example, a compact disc (CD) or magnetic tape may be used to store music (analog audio) simultaneously with a standardized protocol (digital data) to control other equipment (e.g., audio processors, lights) in a home entertainment system or a live performance system. Similarly, a CD player or magnetic tape may interface with a computer to control synchronously, using digital data, events in a computer game while generating audio sounds. Other examples include the display of text (e.g., lyrics, using digital data) with music, the combination of speech and text for educational purposes (e.g., language or reading instruction) or entertainment (e.g., "talking books"), the compression of musical instrument digital interface (MIDI) data with recorded audio, the synchronous control of lights with music using simple lighting control protocols, the synchronization of music with computer-controlled water fountains using a simple control protocol, the control of slides and other audio-visual equipment during a prerecorded presentation, and the transmission of data over analog audio broadcast links. Several techniques have been developed to address the need for combining analog audio signals and digital data signals. One such technique requires that the analog audio signal be converted to a digital audio signal and then combined with the digital data signal (by interleaving digital data bits) to form a single digital composite signal. This is exemplified by Furukawa in U.S. Pat. No. 4,731,774, which discloses a technique for inserting digital data into the digital subcode bits of a CD digital audio bitstream. A disadvantage of Furukawa's technique is that, to retrieve the embedded data, a special CD player having a digital output such as the Sony Philips Digital Interface ("SPDIF") digital output must be used. Furthermore, special hardware must then be used to extract the data from the SPDIF signal. Compared to analog-only systems, such techniques are complicated and expensive to implement.
Alternatively, analog audio signals and digital data signals may be combined by first converting the digital data signal to an analog data signal and then combining it with the analog audio signal to form a single analog composite signal. This has been done on an elementary scale with slide projectors in which an audible tone, for advancing slides, is incorporated into an associated recorded narrative. Two disadvantages of this technique are that the data signal is audible and the data transfer rate is too limited for many of the applications described above.
Another technique that may be used to form a single composite analog signal, developed by a co-inventor of the present invention and disclosed in U.S. Pat. No. 4,672,605, is to use a spread spectrum "data-over-voice" system in which alarm messages and other data are transmitted over telephone wires simultaneously with voice conversations. As in the technique described above for controlling a slide projector, the data signal in the spread spectrum system is audible, and is limited to extremely low data rates.
For the foregoing reasons, there is a need for a cost-effective signal processing system that can record and retrieve, inaudibly and at flexible transfer rates, digital data within an analog audio signal.